It's another hedge fund horror show as Valeant gets slaughtered

The embattled Canadian pharma company confirmed its part in several ongoing investigations. They include ones by the US Attorney's Offices for Massachusetts and the Southern District of New York, the US Securities and Exchange Commission (SEC), and Congress.

The stock collapsed 18.44%, or $14.87, to close at $65.78 per share. The stock hit a 52-week low on Monday of $63.75.

The traumatic start to the week extends a tough run for Valeant, which has been sliding since late 2015 because of scrutiny in Washington, DC, over drug-price increases and accusations from a short seller.

The stock has been a hedge fund darling. According to Goldman Sachs' most recent Hedge Fund Trend Monitor, Valeant ranked No. 26 on its list of the stocks that "matter most" to funds. Valeant had fallen off Goldman's list amid the stock's sell-off in Q3, but it reentered the ranking in Q4.

Here's a quick rundown of the top-10 biggest hedge fund holders, according to regulatory filings compiled by Bloomberg. These are positions the they held for the fourth quarter ending December 31. These funds could have traded in or out of those positions since then:

If they haven't changed their positions, as a group, then they've seen more than $1.25 billion on paper wiped out since Friday's close.

Bill Ackman, founder of Pershing Square, has lost more than $321 million on his position since Friday's close. To date, he's suffered losses estimated at north of $2 billion on his Valeant investment. Others are likely feeling the pain, too.